It is universally accepted that the aging process occurs in all species. Nevertheless, a scientific description of what this process is or what is entailed in the process of aging is almost totally lacking. It has been suggested that aging is related to decreasing activity of certain enzymes. One of the major functions of trace elements is their incorporation into specific enzymes as integral components of that enzyme molecule. The objective of this proposal is to determine trace element content in tissues of the rat from birth to death, as well as the subcellular distribution of these elements in the brain, liver, kidney, heart, and muscle tissue. The elements to be measured are zinc, copper, magnesium, calcium, selenium, and manganese. Various enzymes, dependent on or containing several of these elements, will be assayed in the brain, liver, kidney, heart, and muscles. Differences in the trace element content of these tissues, accompanied by aging, will be discerned on the basis of moisture content, protein content, or concentration per unit of tissue. Additionally, longevity of another group of rats will be increased by underfeeding. These same parameters will be evaluated and compared with the results obtained in the ad libitum group.